1. Field of the Invention
The present invention relates to a large scale power supply system, and in particular to an uninterruptible power supply system providing high quality power through a system including a primary on-site power supply, secondary utility power supply and a rapid switching response to island the facility in the event of utility power quality disruptions.
2. Description of the Related Art
Primary power supply for facility operation and home usage is conventionally provided by government regulated power utilities, such as for example PGandE in northern and central California. It is a goal of utilities to provide low cost, high quality power to service subscribers"" energy needs. High quality power is power that is continuously available at a relatively constant voltage and frequency. Most homes and some small facilities rely solely on utilities for their power needs. However, it is more common, at least in facilities, to employ some sort of emergency backup power generation system in the event the voltage and/or frequency of the utility supplied power fluctuates outside of acceptable ranges, or fails entirely. Emergency backup systems in general minimize health and safety risks to facility personnel and occupants, and ensure continued satisfactory operation of the facility when power supply from the utility is inadequate or fails. Moreover, certain facilities require an uninterrupted supply of power to prevent damage to facility systems and/or the loss of critical data. Such facilities include semiconductor wafer fabs, automated manufacturing plants and those employing data processing systems. Uninterrupted power supplies are also often necessary in a wide variety of facility control systems. Power failure to such facility systems for even a second or two can be extremely disruptive, damaging and dangerous.
There are several conventional emergency backup power supply schemes. A first such system, shown in FIG. 1, employs a battery 10 (or battery bank), a battery rectifier/charger 12 and an emergency AC bus 14. The rectifier/charger 12 is provided for converting the current to the battery from AC to DC and for charging the battery. An inverter 11 is also provided for converting the current leaving the battery from DC back to AC. If power from the utility 16 is inadequate or fails entirely, a control system 18 switches the power feed to the battery. This type of system has the advantage that the power supply may be switched over from the utility to the battery in less than a cycle. However, such systems can only supply power for short periods of time, and are not practical for long term uninterrupted power supply (e.g., more than a few minutes). Moreover, these systems themselves consume a relatively large amount of power and are extremely expensive to operate in megawatt power applications.
Another emergency backup power supply system, shown in FIG. 2, employs a generator 20. The generator includes a motor which burns a working fluid, such as gasoline, natural gas or diesel oil, to produce a mechanical output force. This mechanical output force is then used to rotate conductive coils in the presence of a stationary magnetic field, or visa-versa, to create electrical energy in the windings. If power from the utility 16 is inadequate or fails entirely, the engine runs the generator 20 to produce power, and when the voltage and frequency of the output power reaches acceptable levels, a control system 18 activates a transfer switch 24 to supply the load from the generator. While such systems may run for long periods of time, a significant drawback to backup generators is that they take several seconds until the power can properly supply the load. As such, use of a generator as a backup power system is not feasible for facilities requiring uninterruptible power supply.
Perhaps the most common emergency backup power supply system for uninterruptible power applications is a combination of the systems of FIGS. 1 and 2 to provide an uninterruptible power supply. In particular, upon the voltage or frequency of the utility power supply fluctuating outside of acceptable levels, power is switched to a temporary power supply such as the battery system. The battery system supplies the load until the output from the backup generator is at acceptable levels, at which point the power supply is switched over to the generator. Instead of the battery system, it is also known to have a temporary power supply in the form of a continuously running motor fed by the utility and including a flywheel with a high moment of inertia. If the utility power falls or falls below acceptable levels, the facility load is switched to the motor, which supplies power from the kinetic energy stored in the motor. As the kinetic energy is dissipated, the backup generator is brought on-line, and once the power output from the backup generator is at acceptable levels, the power supply is switched over to the generator. Each of these combination systems requires implementation, maintenance and control of additional systems. Moreover emergency backup systems employing short term supplies (such as the above-described battery and flywheel) are expensive to operate. The short term supply system can often cost approximately two-thirds of the overall cost of the power supply system.
It is also known, as shown in FIG. 3, to have a primary feed 22 and secondary feed 24 to the power utilities. While this is satisfactory for uninterrupted power supply, a second connection to a power utility is expensive to implement and maintain. Moreover, a second connection is often unavailable.
A common feature in known power supply systems is that the primary facility load is supplied by the utility. Moreover, such power supply systems employing emergency backup power supply for megawatt applications conventionally include short term power supply components, which as indicated above are expensive to maintain. There is at present a need for an alternative power supply system capable of supplying low cost, high quality power on demand to facilities which require an uninterruptible power supply.
It is therefore an advantage of the present invention to provide a novel uninterruptible power supply system.
It is another advantage of the present invention to provide low cost, high quality power for megawatt applications.
It is a further advantage of the present invention to provide an uninterruptible power supply that includes no short term power supply, together with an attendant reduction in cost.
It is a further advantage of the present invention that the primary source of facility power supply is located on-site.
It is another advantage to provide a high level of power quality in part through a rapid switching response to island the facility in the event a power quality event degrades the power received from the utility.
It is a still further advantage of the present invention to provide significant cogeneration opportunities for servicing other facility needs.
It is another advantage of the present invention is to allow the power supply to be controlled by the facility instead of the utility.
It is a still further advantage of the present invention that conventional power delivery systems may be easily converted to operate in accordance with the principals of the present invention.
These and other advantages are provided by the present invention which in preferred embodiments relates to a power delivery system including a primary power bus for transferring power to the facility from on-site generators, and a secondary power bus for transferring power to the facility from a utility. The system further includes a static disconnect switch capable of quickly isolating, or xe2x80x9cislandingxe2x80x9d, the facility from the utility power grid, and a controller for controlling the overall operation of the power delivery system.
The utility preferably provides a small, constant power flow to supplement the primary generator power supply in a normal mode of operation. In setting the precise relative contributions of the on-site generators and utility in a normal mode of operation, the utility power supply should be low enough that the generators can handle the facility islanding at any time upon a drop in power quality from the utility. At the same time however, the utility power supply should be high enough to prevent frequent tripping of the reverse power relay (at least in the United States) and to prevent acceleration of the generators upon a drop in facility load.
In a normal mode of operation, these factors are balanced by the utility supplying power approximately in the amount of the single largest load in the facility, with the remaining power being supplied by the on-site generators. Setting the power draw from the utility less than this presents the risk that power will attempt to flow into the utility power grid in the event that high-load facility component goes off-line. Setting the power draw from the utility much higher than this presents the risk that the generators will be unable handle the initial load upon a drop in power from the utility and islanding of the facility.
If a generator goes off-line, the additional power may come from the other generators, in the event a redundant generator system is employed, or the additional power may come from the utility. In the event a redundant generator system is used, the controller controls the operation of the generators to make up the power from the off-line generator. Where there are no redundant generators, or during the time that the redundant generator or generators are brought on-line, additional power is automatically drawn from the utility to supply the load without separate protocols in the controller. In the unlikely event that each of the generators goes off-line, 100% of the facility load may be supplied by the utility.
Should there be a drop in the quality of power from the utility, this event is detected by sensors in communication with the static disconnect switch controller, and the switch opens in less than a cycle to thereby island the facility from the utility power grid so that 100% of the facility load is supplied by the on-site generators. This protects the facility against utility power quality disruptions and the rapid switch over allows the power supply system according to the present invention to continuously provide a high level of power quality to the facility.